Modern, wind-driven electricity generators were born in the late 1970′s. Until the early 1970s, wind energy filled a small niche market supplying mechanical power for grinding grain and pumping water, as well as electricity for rural battery charging. With the exception of battery chargers and rare experiments with larger electricity-producing machines, the windmills of 1850 and even 1950 differed very little from the primitive devices from which they were derived. As of July 2014, wind energy provided approximately 4% of total U.S. electricity generation. Most modern wind turbines typically have multi-bladed rotors with diameters of 10-80 meters mounted atop 60-80 meter towers. Another known turbine design is known as a “rimmed” turbine, in which significantly more than three blades are mounted to an in inner hub and at their inboard end and an outer rim at their tips. This type of turbine has been used extensively in rural farming for pumping irrigation water for decades. In both cases, the blades can extend substantially equidistantly around the axis of rotation. Each blade has an aerodynamic shape selected to exert a rotational torque in the presence of wind that rotates the blade about an axis of rotation at the geometric center. The blades are shaped to cause this rotation in response to a prevailing wind moving generally parallel to the axis of rotation.
The relationship between wind speed and forces on the mechanical components are based on a relationship of y=x3. Thus, relative to the stress caused by a 15 mph wind, a 30 mph wind induces 8 times more stress, a 60 mph wind causes 64 times more stress, and a 120 mph wind causes 512 times more stress. Certification of a wind turbine in Hawaii requires that they must be able to withstand occasional hurricane force winds and must be 512 times as strong as a wind turbine would need to be if it only encountered an average wind speed equal to 15 mph, or roughly category 3 hurricane levels. Prior art wind turbines either (a) must be therefore significantly overbuilt to withstand damaging storm winds that occur during a very small percentage of their useful life, or (b) are limited to impractical placement in the very few geographical areas that will not experience damaging winds.
The prior art includes various designs to change the orientation of the blades relative to the wind to reduce that stress. For example, Applicants' US Patent Publication 2010-0140949 shows a design where the individual blades can be retracted into a closed position. U.S. patent application Ser. No. 12/461,719 incorporated herein shows a design in which the enter wind turbine is simply moved into a horizontal plane where the blades do not face into the wind.